Air-backed diaphragm pumps provide an economical and reliable solution for many fluid handling applications. However, they are not suitable for use in a system where a precise or highly stable flow rate is required. The flow rate of such pumps drops with increasing pressure on the pump outlet for several reasons. First, the increased load slows the motor down resulting in a reduced flow rate. Second, the unsupported regions of the pump diaphragm balloon slightly in proportion to the pressure on the outlet, reducing the effective displacement and flow rate of the pump. These diaphragm distortion-based flow rate losses are independent of motor type.
While load induced motor speed reductions can be compensated by using control techniques well known in the art, such correction techniques only counteract about 70% of pressure induced flow rate losses. These techniques do not compensate for diaphragm distortion-based flow rate losses which can account for up to 30% of the pressure induced flow reduction.
One method of eliminating the distortion based diaphragm flow rate loss is to support the diaphragm with some incompressible liquid in a sealed chamber behind the diaphragm. However, such liquid-backed pumps are inefficient and relatively expensive compared to air-backed pumps. Alternatively, a rotor vane pump can be used instead of a diaphragm pump. A rotor vane pump can accurately maintain a specified flow rate through varied outlet pressures. However, rotor vane pumps are very expensive. Moreover, rotor vane pumps cannot pump fluids containing large particulates (&gt;50 micron) and as a result require the use of micro filters to screen any fluid that is pumped. Air-backed diaphragm pumps, on the other hand, are inexpensive and can pump fluids with relatively large particulates without impairing the pump.
An object of the invention is to provide a low cost metered flow pumping system that provides substantially constant flow rate across a range of pump output pressures.
Another object of the present invention is to provide a metered flow pumping system that is not sensitive to the presence fluid particulates.
Another object of the invention is to provide a metered flow pumping system using an air-backed diaphragm pump.